In the manufacture of semiconductor devices, the surface of semiconductor wafers must be cleaned of wafer contaminants. If not removed, wafer contaminants may affect device performance characteristics and may cause device failure to occur at faster rates than usual.
One system used to remove wafer contaminants is commonly referred to as a scrubber. In at least one type of scrubber, a semiconductor wafer is scrubbed simultaneously on both sides by brushes. While the wafer is being scrubbed simultaneously on both sides by the brushes, the wafer is held in place and rotated so that the entire surface of the wafer is cleaned. Rollers may be used for this purpose.
Scrubbers usually comprise a conveyor type mechanism, rollers, and brushes. In general, the wafer lies flat on the conveyor mechanism and the conveyor mechanism moves the wafer into the brushes. While being scrubbed, the wafer is supported (or held horizontally) by the conveyor mechanism, brushes, rollers, or a combination thereof. In one prior art type of scrubber, as the wafer is being scrubbed by the brushes, the roller rotates the wafer so that the entire wafer surface may be cleaned. The roller itself is being rotated about its central axis by a motor. The rotary motion of the roller is transferred to the wafer when the edge of the roller comes into contact with the outer edge of the wafer.
FIG. 1 illustrates a prior art roller. Referring to FIG. 1, the top and bottom surfaces 110 and 120 of the roller 100 are generally flat and the outer edge 130 of the roller 100 has a slight concave indentation (concave outer edge). As is shown in FIG. 1, the concave outer edge of the roller 100 contacts the outer edge of the wafer 150. When the roller 100 and wafer 150 contact each other, friction between the edges is created, so that the rotation of the roller 100 causes the wafer 150 to rotate. As shown in FIG. 1, the wafer 150 and roller 100 are in essentially single point contact.
One problem with this prior art roller is that the roller/wafer contact may be insufficient, so that the wafer hesitates, i.e. fails to rotate. If the wafer fails to rotate during the scrub operation, some areas of the wafer will not be scrubbed, so that the wafer is not cleaned to the desired level. An additional problem which may occur is that if the system relies on the roller to rotate the flat into a predetermined position for the next operation, the wafer flat may be misplaced. For example, after the scrub operation, the wafer may next go to a spin dry operation where it is held by its edges. If the wafer flat is misplaced, the wafer may not be held properly.
FIG. 2 illustrates another prior art roller that compensates for insufficient contact. Referring to FIG. 2, the roller 200 includes top and bottom surfaces (201, 202) which are generally flat, slightly indented portions 203 and 204, and an inner groove (groove) 210. When a wafer, such as wafer 250, is being cleaned between brushes, it is pushed forward and inserted into the groove 210 of the roller 200 to a point where the groove 210 pinches the wafer 250 causing increased contact, and therefore, increased friction between the roller 200 and the edge of the wafer 250. Thus, when the roller 200 is rotated the friction causes the wafer 250 to rotate.
However, even though prior art rollers provide a mechanism to pinch a wafer, slippage between the wafer and the roller can still occur when cleaning solutions, such as ammonium hydroxide (NH.sub.4 OH) and/or water, are used in the cleaning process. The NH.sub.4 OH or other cleaning solution acts as a lubricant between the outer edge of the wafer and the outer edge of the roller, reducing the friction between the edges when they contact and causing slippage. It is desirable to reduce the slippage that may occur between a wafer and a roller when solutions are used in the cleaning process.
The treads allow better contact between wafer edge and roller as well as allow channels for fluid to drain through. (This is as important as the flow of fluids).
Another problem that occurs when scrubbing both sides of a wafer at the same time is that the points of contact between the brushes and the wafer may not be aligned vertically. During the scrubbing process, the application of the brushes results in pressure being applied to the wafer at their point of contact. When these points of contact are not aligned vertically, the wafer may be tilted from its horizontal position due to pressure being applied on the wafer by one or both of the brushes. The tilting of the wafer may cause the wafer to be displaced from the roller(s), and even shoot over the roller(s). It is desirable to maintain wafer contact with the roller during the scrubbing process when points of contact between the brushes and the wafer are not vertically aligned.
What is needed is a method and apparatus for rotating wafers in a scrubber with reduced or eliminated slippage, particularly when cleaning solutions are being used.